Strategies for finding biological grandparents using DNA matches?

Question

I need some advice to help track down my biological grandmother. My father was adopted in 1920 and died in 2000 without knowing the identity of his biological parents. I have his birth certificate but I am not certain that the name of his mother is her real name. I DNA tested with Ancestry and found 1000+ matches. To my surprise, one shared 1752 centimorgans with me. By process of elimination, he has to be a hitherto unknown half-brother. I am certain he is on my father’s side because none of my cousins on my mother’s side who tested with Ancestry share matches with him. I have tried to contact him but have not heard from him. It would be nice if I could have him upload his raw data to GEDMatch but I do not expect to hear from him. I did find matches for people at 222, 169, 140, 64, and 60 centimorgans all of whom were kind enough to allow access to their family trees. By filling out my tree with their information I was able to locate my paternal grandfather. All of these people fell into the proper place on the genealogy chart based on the centimorgans we share.

On Ancestry there are 5 matches I know to be maternal cousins. I used them to eliminate shared matches leaving what should be paternal matches. I’m not sure if I am on the right track but I also eliminated those that matched with the half-brother leaving matches that, theoretically, should only be to my paternal grandmother’s family.

Am I on the right track here or am I missing something?

Additionally, how should I use the information about the half-brother to trace up family trees and back down to find my grandmother?

I’m just not sure how far back to go or what relationship to look for.

Answer 1

Using your five known maternal matches to separate out most of your maternal matches from your paternal matches is definitely the correct track.

However, further eliminating those that match with your paternal half-brother does not reduce your remaining matches to those who match your paternal grandmother. Your remaining matches are potentially both maternal and paternal, and just don't match either your five maternal cousins or your paternal half-brother. Remember, your paternal half-brother's paternal grandparents are the same people as your paternal grandparents.

How did you determine that you found your paternal grandfather? Why couldn't one of that man's sisters have been your paternal grandmother instead?

Depending on the state in which your father was born/adopted, you may be able to request an original birth certificate (OBC). The birth certificate will minimally have a name for the mother, though this was sometimes a fake name. The best place to review whether the state may allow you to request an OBC is the American Adoption Congress.

You might be able to find other matches who are willing to help you by uploading your raw DNA data to sites like FamilyTreeDNA, GEDmatch, and MyHeritage.

Answer 2

Yes, you're on the right track and doing a great job so far. The only thing I see wrong in your logic is that people who don't match your half-brother should be on your mother's side, not your paternal grandmother's side. Your half brother is still fully related to both paternal grandparents.

What you have been doing so far is akin to what's called Chromosome Mapping. Look it up and use the free tool www.dnapainter.com. Your goal is to find people who match you on your paternal grandmother's segments. Then look up the ancestry of those people for surnames or places in common and maybe putting their trees together you can narrow down who your grandmother is.

You'll need to go back far enough in the trees to identify the most recent common ancestor, so if you match 2nd cousins, then 4 generations back. 3rd cousins require 5 generations back, etc.

Answer 3

The conclusion in this statement isn't quite right:

On Ancestry there are 5 matches I know to be maternal cousins. I used them to eliminate shared matches leaving what should be paternal matches.

When you remove your matches with a maternal cousin, you are only removing some of the maternal relation matches you have. The set of remaining matches are a bit more likely to be paternal, but many of them will remain maternal.

When you remove the matches with another maternal cousin, you will again be enriching the percentage of paternal matches in the remaining set of matches. However, in general you cannot expect to eliminate all the maternal matches of the set. This is because you got some maternal DNA that none of your maternal cousins got, and you may well have matches on that "unique" maternal DNA that no known cousin will match.

So you will not be leaving only paternal matches. The ones left will more likely be paternal, but some will be maternal.

I also want to reinforce what lkessler said: the matches you share with your half brother are from both paternal grandparents, not just your paternal grandmother. With the information you have, you can't determine which matches belong to which.

You should not eliminate your half brother's shared matches, but rather focus on them, as those are the ones most likely to yield results even though about half of them will be from your paternal grandfather rather than your paternal grandmother.